Method of patterning Cr-Pt-Au metallization for silicon devices

ABSTRACT

The disclosure relates to patterning of platinum effectively and then removing the mask from the platinum without attacking the chromium metal beneath the platinum. If tantalum is used on top, which metal can be patterned with CF 4 , the tantalum can be patterned and the platinum can then be patterned quite readily and the tantalum can then be removed with the CF 4  again and then the gold can be plated up onto the platinum patterns and the chromium can then be etched out using conventional chromium etch which will not attack any of the other metals. 
     This solves the problem of high yield patterning with a metal mask and process for patterning and removal of that metal mask that is compatible with the chromium-platinum metal system. The advantage of the chromium-platinum system over the titanium-platinum system is the resistance of the chromium to silicon etch. Therefore, the principal application of this invention would be in the fabrication of mesa devices.

This is a continuation, of application Ser. No. 502,472, filed Sept. 3,1974, now abandoned.

This invention relates to a method of patterning Cr-Pt-Au metallizationfor silicon devices and, more specifically, to a method of effectivelypatterning platinum over chromium without removing the chromium whenremoving the mask required for patterning the platinum.

In using the Cr-Au process for making mesa diodes, platinum is requiredbetween the chromium and the gold to make the system metallurgicallystable so that in reality there is a chromium-platinum-gold system. Thegold itself is deposited by electro deposition onto the platinum. To dothis, it is necessary, to first pattern the platinum. Patterningplatinum is difficult because it does not etch well and is etched onlyby aqua regia. In the present state of the art, this is done by usingconventional photoresist, but the yield is not optimum because the aquaregia that patterns the platinum will also attack the photoresist. Thisinvention is primarily to allow patterning of the platinum effectivelyand then removing the mask from the platinum without attacking thechromium metal beneath the platinum. The chromium metal must be leftbeneath the platinum because it is necessary to electro deposit, so itis necessary to have island patterns of platinum with an equi-potentialplane of chromium thereunder which is not patterned. Chromium can beused for patterning the platinum, but when the chromium is removed fromthe top of the platinum to electro deposit the gold, the chromium isalso removed from the bottom layer and therefore there would not be anyequi-potential plane. If tantalum is used on top, which metal can bepatterned with CF₄, the tantalum can be patterned and the platinum canthen be patterned quite readily and the tantalum can then be removedwith the CF₄ again and then the gold can be plated up onto the platinumpatterns and the chromium can then be etched out using conventionalchromium etch which will not attack any of the other metals. So theentire system is etch compatible in that what is used to pattern thetantalum does not attack the platinum or the chromium; what is used topattern the platinum does not attack the tantalum or the chromium; andfinally, that which is used for removal of the tantalum does not attackthe platinum or the chromium. When the gold is plated up and thechromium is removed, what is used to remove the chromium does not attackthe platinum or the gold that is left behind.

This solves the problem of high yield patterning with a metal mask andprocess for patterning and removal of that metal mask that is compatiblewith the chromium-plating metal system. The advantage of thechromium-platinum system over the titaniumplatinum system is theresistance of the chromium to silicon etch. Therefore, the principalapplication of this invention would be in the fabrication of mesadevices.

It is therefore an object of this invention to provide a method ofpatterning platinum over a copper layer without affecting the copperlayer.

It is a further object of this invention to provide a method ofpatterning Cr-Pt-Au metallization for making mesa diodes.

It is a yet further object of this invention to provide a tantalum maskover a Cr-Pt system for patterning of the platinum.

The above objects and still further objects of the invention willimmediately become apparent to those skilled in the art afterconsideration of the following preferred embodiment thereof, which isprovided by way of example and not by way of limitation, wherein:

FIGS. 1 through 3 are cross-sectional views of a semiconductorstructure, illustrating successive stages in fabricating the preferredembodiment of the invention.

Referring first to FIG. 1, there is shown a semiconductor wafer 11,preferablly formed of silicon, though other semiconductor materialscould be used. The wafer 11 is processed by any suitable technique toprovide therein the indicated region of N-type and P-type conductivity.The structure illustrated is a portion of an integrated circuitstructure to be provided with the necessary metal interconnectionsystem. An N-P-N transistor is formed by emitter 12 and base 13 incombination with substrate 11 which serves as a collector region. P-typeregion 14 is a resistor element to be connected electrically withtransistor base region 13. An insulating layer 15, typically silicondioxide, for example, covering the surface of wafer 11, acquires astepped configuration, as shown, during the successive diffusionoperations. Thereafter, openings or windows are sleectively etched inthe oxide layer to permit ohmic contract with each of the respectiveconductivity regions by the first metallization level. Also, the siliconsurface exposed by the ohmic contact holes in oxide layer 15 are coveredwith a layer of platinum silicide (not shown), if desired, for thepurpose of improving ohmic contact with the first metallization level.

Chromium layer 16, platinum layer 17 and tantalum layer 18 are depositedby known techniques, including, for example, vacuum evaporation orsputtering. The tantalum may be deposited in the same or subsequentpump-downs to that used with the chromeplatinum deposition. Tantalumlayer 18 is then patterned, using photolithographic masking and aplasma-vapor etch in CF₄, this being a selective etch to provide theappropriate pattern for use as a mask in the subsequent etching ofplatinum layer 17. For example, the tantalum layer 18 is first coveredwith a suitable photoresist composition which is selectively exposed toa suitable light source and then developed to provide the exact patternrequired. A suitable etch for the removal of the exposed tantalumpattern is CF₄ as noted above. After removal of the tantalum with CF₄,the structure will be as shown in FIG. 1.

The structure shown in FIG. 1 is then immersed in aqua regia for thepurpose of removing the exposed portions of the platinum layer 17. Theaqua regia does not appreciably attack the tantalum mask or chromiumlayer 16 due to the passivating effect of oxides initially formed uponreaction with the nitric acid contained in aqua regia. After the aquaregia etch, all other resists are removed and the tantalum layer 18 isremoved by plasma-vapor etch in CF₄. The chromium layer 16 and theplatinum layer 17 will not be attacked by the CF₄ plasma-vapor etch.

Referring now to FIG. 2, there is shown the resulting pattern formed inthe platinum layer 17 with the chromium layer 16 thereunder. Aphotoresist pattern 19 is then applied at all points where the chromiumlayer 16 is exposed to the upper surface as shown in FIG. 2 with thepatterned platinum region 17 being exposed to the surface with nophotoresist thereon. A layer of gold 20 is then plated onto the platinumregions 17 to a thickness compatible with device or circuit requirementsusing solutions and techniques well known in the art. Techniques such asvacuum evaporation or sputtering can be used. The photoresist layer 19is then removed by well known means and the chromium layer 16 is thenetched in the exposed region using the platinum-gold layer as a mask andusing etches based on ceric sulfate or ceric ammonium nitrate. Forexample, the solution can be prepared from 50 mililiters of 0.5 N cericsulfate in 2N sulfuric acid and 35 mililiters concentrated nitric aciddiluted with water to yield 135 mililiters of etching solution.Hydrochloric acid solutions may be used to etch chromium, however, theyare generally inferior to the ceric reagents.

Referring now to FIG. 3, there is shown a structure after thephotoresist layer 19 has been removed and the chromium layer 16 has beenetched as set forth hereinabove. It can be seen in FIG. 3 that in thoseregions where the chromium layer 16 and the SiO₂ layer 15 have beenremoved, the silicon layer 11 is exposed to the upper surface. The oxidelayer would be removed in regions where silicon etching is to be done.The chromium-platinumgold layers 16, 17 and 20 will serve as an etchmask for the HNO₃ --HF base silicon etch which would now be used toprovide any etching in the region 11.

It can be seen that there has been provided a method which solves theproblem of high yield patterning of platinum which a metal mask processfor patterning and removal of that metal mask that is compatible withthe Cr-Pt metal system. The advantage of the Cr-Pt system over the Ti-Ptsystem is the resistance of the chromium to silicon etch. Therefore, theprincipal application of this invention would be in the fabrication ofmesa devices.

Though the invention has been described with respect to a specificpreferred embodiment thereof, many variations and modifications willimmediately become apparent to those skilled in the art. It is thereforethe intention that the appended claims be interrupted as broadly aspossible in view of the prior art to include all such variations andmodifications.

What is claimed is:
 1. A method of forming a patterned platinum layerover a chromium layer which comprises the steps of:(a) forming a layerof platinum over said chromium layer, (b) forming a layer of tantalumover said platinum, (c) patterning said tantalum by a plasma-vapor etchto form a mask over said platinum, (d) patterning said platinum throughsaid tantalum mask, (e) removing the remaining tantalum by a CF₄plasmavapor etch, whereby the initial chromium layer is left intact dueto the selectivity of the CF₄ plasma-vapor etch for tantalum, and (f)selectively plating gold onto the platinum.
 2. A method as set forth inclaim 1 wherein said layers are formed on a semiconductor slice.
 3. Amethod as set forth in claim 2 wherein said tantalum layer is patternedusing a CF₄ etch.
 4. A method as set forth in claim 2 wherein saidplatinum layer is patterned using aqua regia.
 5. A method as set forthin claim 3 wherein said platinum layer is patterned using aqua regia. 6.A method as set forth in claim 1 wherein said tantalum layer ispatterned using a CF₄ etch.
 7. A method as set forth in claim 6 whereinsaid platinum layer is patterned using aqua regia.
 8. A method as setforth in claim 1 wherein said platinum layer is patterned using aquaregia.